I have been developing 35 mm film for a while. Recently, I had a fixer go bad (foggy negative) so I mixed a new batch from the same concentrate.

Today, I also mixed a new batch of dev powder (Xtol, standard concentration) and developed an Ilford HP5+ 400. No push/pull.

Click to expand...

A thought on your developer. I have known beginners mix half a pack of developer powder and then use the second half of the powder to make a second batch of developer. Developer mixes will separate into their constituent ingredients with handling and it is important to use the whole pack at one time to get the correct make-up. If you have tried that, it would explain your results.

I agree with others that it is the developer as the film edge markings are not as they should be.

Another potential problem is using too much water (easier to do than you might think!) when making up the developer.

Further thoughts: batteries in the camera/meter becoming flat - happens with alkaline batteries before they stop working. ISO setting on the camera/meter getting inadvertantly moved. If a modern film camera, exposure compensation accidentally set. Lens set to manual rather than auto (old M42 cameras).

Xtol. High risk developer. One never knows, when death will come. It is not unknown for being bad right from the package. Long time ago Kodak stopped making Xtol in small 1 liter packages as it was notorious for being bad right after mixing. Large packages, 5 liters, have also limited shelf life. The guilty part is vitamin C, when goes bad lowers the pH too much from initial pH 8 to in this case to barely above 7. In such a pH metol is almost inactive.
BTW bad fixer will not make negative foggy, but may cause image deterioration later, gradually and usually in form of stains.
I don't know, what do you mean by while (I have been developing 35 mm film for a while), I am saying the same about myself. My "while" started in 1975 to give some perspective to it. I learned in my time, that the best tool to avoid or analyze mistake if happens is knowledge. In a case here:Mixing Developers
is one of the most condensed "knowledge pills" about developers. And there is more, much more. Whole 150 years of film photography history, all available to you in case film photography is not only a fashion statement for you.
Cheers.